The present invention relates to frequency synthesis and, in particular, to primary frequency synthesizer combined with a programmable offset frequency synthesizer.
The demand for wireless communication services has grown dramatically in recent years. A major problem facing the wireless communications industry is the limitation of available radio frequency spectrum for wireless communications. Originally, a 50 MHz portion of the radio frequency spectrum was allocated for the Advanced Mobile Phone System (AMPS). AMPS uses Frequency Division Multiple Access (FDMA) to create 30 kHz radio frequency channels within this 50 MHz spectrum. Each radio frequency channel supports a single call within a geographic cell. Since the available bandwidth limits the number of available channels, the number of users that can be simultaneously supported using FDMA is severely limited.
As demand for wireless communication services has continued to grow, new digital processing techniques have been devised to more efficiently use the available spectrum and to increase system capacity. In a digital communication system, the analog speech waveform is first digitized and then compressed prior to modulation to decrease the data transmission time. These digital processing techniques allow several voice users to be multiplexed over a single radio frequency channel, which increases the cellular system capacity without increasing the bandwidth.
One multiple access technique in common use today is called Time Division Multiple Access (TDMA). TDMA is a multiple access scheme that allows multiple users to share the same radio frequency (RF) channel. Each RF channel is divided into periodic xe2x80x9cframesxe2x80x9d with each frame subdivided into several equal duration time xe2x80x9cslots.xe2x80x9d Each slot serves as a communication channel. A mobile terminal is assigned a slot in the frame during which the mobile terminal transmits and receives information in short bursts. Since there are several slots per frame, a plurality of mobile users can simultaneously use each radio frequency channel. Thus, each radio frequency channel can support multiple communication channels. An exemplary TDMA scheme is standardized by the Electronics Industries Association (EIA) and the Telecommunication Industry Association (TIA) as TIA/EIA-136, which is incorporated herein by reference.
In a typical TDMA system, separate frequencies are used for downlink communications (base station to mobile terminal) and uplink communications (mobile terminal to base station). For each downlink frequency, there is a corresponding uplink frequency. The downlink frequency and its corresponding uplink frequency are referred to herein as a frequency pair. In conventional TDMA systems, such as TIA/EIA-136 and the European Global System for Mobile Communications (GSM), spacing between all downlink and corresponding uplink frequencies is fixed. For example, uplink and downlink frequencies for a given communications channel pair are offset by 80 MHz in GSM-based systems.
When a call is established, the mobile terminal is assigned a slot on both uplink and downlink frequencies. Assigned uplink and downlink time slots represent a communication channel. The assigned slots are then xe2x80x9cownedxe2x80x9d by that mobile terminal until the call ends unless the mobile terminal is reassigned to a different channel (e.g., hand-off). During the call, the assigned time slots cannot be used by any other mobile terminals.
Even with TDMA and other multiple access schemes, the high growth in demand for wireless communications services will soon exceed the capacity of existing mobile communications systems. No one expects that the growing demand for cellular services can be met by allocating additional spectrum. Therefore, the current challenge is to meet the demand by increasing system capacity within the spectrum already allocated.
Recently, a technique called statistical voice multiplexing has been proposed to increase the capacity of a mobile communication system. Statistical voice multiplexing exploits the intermittent nature of human speech to improve spectral efficiency of a mobile communication system. When engaged in normal conversation, humans alternately assume speaking and listening roles. Even when speaking, human speech contains xe2x80x9ctalkspurtsxe2x80x9d interspersed with numerous pauses during which no information is conveyed. Next-generation communication systems could potentially employ statistical voice multiplexing techniques that enable communications systems to exploit the intermittent nature of human conversation. Using statistical voice multiplexing, uplink and downlink slots are reassigned during pauses in speech. For example, a particular mobile terminal user might be at a point in conversation where he or she is listening to the far-end caller. When the mobile terminal is not transmitting information on its assigned uplink channel, the base station may reassign this uplink channel to another mobile terminal that actually needs it. When the user of this particular mobile terminal begins speaking, the base station must quickly assign an available uplink channel to it.
Using statistical voice multiplexing, the mobile terminal may change uplink and downlink communication channels many times during a single call. Because the uplink and downlink time slots are assigned based on availability, there is no longer a fixed offset relationship between uplink and downlink frequencies. Instead, all uplink and downlink time slots on each of the defined uplink and downlink frequencies are treated as a common pool of communication channel resources that may be dynamically assigned and reassigned among all of the active mobile terminals based on the actual needs of each mobile terminal.
Statistical voice multiplexing techniques require that each mobile terminal be able to quickly change its downlink and uplink frequencies many times during the course of a single call. The base station may require the mobile terminal to change only its uplink frequency, its downlink frequency, or both, depending on overall system needs. Unlike prior systems where the uplink frequency was related to the downlink frequency by a defined fixed offset, these newer generation communication systems require the mobile terminal to generate uplink frequencies over a range of defined offsets from the downlink frequency. This requirement increases the complexity and cost of the frequency synthesizers used by mobile terminals to generate the operating frequencies associated with assigned uplink and downlink frequencies. Therefore, there is a need for improvements in frequency synthesizers that can generate uplink and downlink frequencies that do not have a fixed relationship to one another.
The present invention provides methods and apparatus for producing two frequencies having a variable offset frequency relationship. A frequency synthesizer provides a first output signal having a frequency determined by a first control signal and a second output signal having a frequency offset from the first output signal as determined by a second control signal. A main synthesizer responsive to the first control signal produces the first output signal. A programmable offset frequency synthesizer responsive to the second control signal produces a variable frequency offset signal. A mixer combines this offset signal with the first output signal to produce the second output signal. As such, the second output signal has an offset frequency relationship with the first output signal, controllable based on changing the second control signal. Likewise, changing the first control signal results in a corresponding change of frequency in the first output signal. Thus, the absolute frequency value of the first and second output signals depends on the first control signal, while the offset frequency relationship between the first and second output signals depends on the second control signal.
In an exemplary embodiment, a mobile terminal incorporates the frequency synthesizer of the present invention. A logic unit within the mobile terminal sets the first control signal to a value corresponding to a downlink (receive) communications channel assignment and sets the second control signal corresponding to an uplink (transmit) communications channel assignment. A receiver within the mobile terminal uses the first output signal to downconvert the received signal for subsequent processing. A transmitter within the mobile terminal uses the second output signal to generate a carrier signal used to transmit signals back to an associated base station on the uplink channel. As the frequency synthesizer of the present invention allows the mobile terminal to independently change uplink and downlink frequencies, the mobile terminal is capable of responding to changing uplink and downlink communications channel assignments as required in wireless communications systems employing statistical voice multiplexing techniques.